As good as Mascherano has been during his FCB time (since he started to play more in the defence than in his natural position as a DM that is) he is not a natural CB and we saw that against RM and Chelsea. Yes, the team was not particularly good in any of those 3 games but it is under such circumstances that world class CB's can show their worth and determine whether the opposition team scores or not.
We definetely need a new CB and we should have already bought one last year. I and several others wanted that to happen but overall we could not complain as players such as Sánchez and Cesc were bought and Mascherano proved himself as a CB in the biggest stage (CL-final)
Personally I would prefer Hummels, Silva, Sakho or Yanga-Mbiwa. Someone who is young but experienced and proven on the highest level, physical strong, quick and with a good technique and reading of the game.
Obviously there are not many players who have all that and those you have are not necessarily good enough to play for FCB but if I should mention one player who would be a brilliant option and probably a successful one, then it would be Thiago Silva. And yes, players from the Italian league have not always shown the same worth in the Spanish league (and vice versa) but I am pretty sure that Thiago Silva would do the job.
Although it is completely unrealistic my own personal favorite is Raphael Varane who I have always praised on the forum even before he moved to RM. People laughed when I said that he could become better than Ramos and Pepe in the upcoming years.
Fantastic talent and just the type of player we could use as well and 9 years younger than Silva...